While it is generally accepted that heterosexual transmission of AIDS from women to men and from men to women has occurred, the degree to which transmission varies by type of sexual activity, and the frequency of sexual contact between infected and uninfected heterosexuals are unknown. Also unknown are the prevalence of infection among prostitutes and the extent to which infection of prostitutes is due to drug abuse or sexual activity. The lack of such information makes it difficult to predict the future rate of growth of the AIDS epidemic or to plan educational programs or intervention strategies in the male and female heterosexual populations. The aims of this project thus are (1) to monitor the prevalence of infection with human T lymphotropic virus type III/lymphadenopathy associated virus/AIDS-related virus (HTLV-III/LAV/ARV) among female prostitutes and heterosexually active males and females; (2) to determine the extent to which infection of female prostitutes is due to drug abuse versus sexual activity; and (3) to investigate risk/protective factors associated with development or prevention of HTLV-III/LAV/ARV infection in heterosexual men and women. Risk/protective factors to be studied include numbers of sexual partners, types of sexual activities, sexual contact with members of AIDS risk groups, protective effects of contraceptive devices, sexual contact with prostitutes; and vaginal or penile lesions. Subjects will complete epidemiologic questionnaires and will donate samples of blood that will be analyzed for HTLV-III/LAV/ARV antibodies using an enzyme-linked immunosorbent assay with Western blot confirmation. Periodic analyses will be conducted to determine the rate of seropositivity in the sample population by age, sex, race, drug use, AIDS risk group and other factors. Seropositive heterosexual subjects will then be matched on age, race and sex to seronegative heterosexuals subjects using a 4:1 matching ratio to conduct case-control analyses. Univariate and multivariate logistic regression techniques will be used to separately investigate risks in males and females. The proposed sample size of about 1,938 males and 1,479 females should provide a power of about 80% (at an alpha significance level of 0.05) in each of 3 sampling periods for males and 2 sampling periods for females to detect risk ratios of 5.0 or greater. Combined analyses of data accumulated in all sampling periods should permit still greater sensitivity.